1. Field of the Invention
The present invention generally relates to method and apparatus for providing a copy of an image available in electronic form and, in particular, to method(s) and apparatus for providing a hardcopy of an image which has been produced by, for purposes of illustration and without limitation, medical imaging equipment such as x-ray equipment, CAT scan equipment, MR equipment, ultrasound equipment, and the like.
2. Description of the Prior Art
A hardcopy has been defined, for example, in an article by D. G. Herzog entitled "Hardcopy Output of Reconstructed Imagery," J. Imaging Technology, Vol. 13, No. 5, October, 1987, pp. 167-178, as "an image that is visible to the human observer, that has a degree of permanence, and can be transported and handled without deterioration of the image. Hardcopy normally is an image imprinted on transparencies where the image is viewed by passing light through the medium or an opaque material where the image is viewed by reflecting light off the image." Many attempts have been made by workers in the field to fabricate apparatus which can make a hardcopy of an electronically generated or stored image.
It is well known that devices for providing hardcopies typically receive image information as output from an image data source such as, for example, a group of sensors, a computer image processing system, or storage devices hardcopy services. Although such may receive image data in either analog or digital form, the general trend in the art today is to receive image data in digital form. Further, such devices typically comprise buffers, memories, look-up tables, and so forth for: (a) electronic processing and/or formatting input image data and (b) modifying the apparatus transfer function to compensate for effects such as, for example, print medium nonlinearities or to compensate for, or to provide, image contrast enhancement. Still further, such hard devices typically comprise an image generator subsystem which includes energy shaping mechanisms and supporting electronics to convert an energy source such as, for example, a laser beam or a CRT beam into focused spots for scanning onto a medium.
There are certain important image quality parameters which must be taken into account when designing a hardcopy device. A first, important image quality parameter is resolution. Most imaging devices have the capability of recording many thousands of picture elements (pixels) across the medium. The ability to distinguish individual pixels or to smooth the image between pixels is determined by the resolution specification. A second, important image quality parameter is raster and banding. Raster and banding are artifacts that usually appear in pixel by pixel recording systems.
Raster is caused by incomplete merging of scan lines and appears as a regular pattern of density modulation at the pixel spacing whereas banding is caused by nonuniformity of pixel placement on the medium and may appear as regular or random patterns of density variation in across-scan or along-scan directions. The appearance of banding depends on the source of placement errors, and since the human visual system is very sensitive to placement errors, placement errors on the order of 1% can be discerned. As a result, banding requirements must be carefully considered due to the cost implications of providing precise pixel and scan line placement.
A third, important image quality parameter is geometric fidelity. Geometric fidelity specifications define the precision with which pixels are located on the medium and relate to how the medium will ultimately be used.
A fourth, important image quality parameter is density fidelity. The density fidelity specification defines the transfer function of the input digital value (or analog voltage) to output density. This specification encompasses the transfer function of value to density and the transfer function of any duplicating process utilized. The transfer function is dependent on processing variables as well as on the nature of the specific medium used. The density fidelity specification can be separated into four parts: (a) absolute density repeatability; (b) relative-density versus input-signal transfer function; (c) area modulation versus continuous tone recording; and (d) density uniformity. The first of these parts, absolute density repeatability, is the ability of the hardcopy device to consistently produce the same density values for given input signals. The second of these parts, relative-density versus input-signal transfer function, i.e., tone scale, is related to the fact that in some applications a linear-density versus input-signal transfer function is utilized while in others a deliberate distortion of the transfer function is utilized to provide contrast adjustment, compensation, or enhancement in certain parts of the density range. The shape of the relative-density versus input-signal transfer function can be adjusted using calibration look-up tables located in a digital input signal processing path, and these tables can be either fixed, locally adjusted via panel controls, or remotely loaded via a control interface. Further, if the shape of the relative-density versus input-signal transfer function is critical, an operational scenario involving media processor control, periodic transfer function measurement, and periodic calibration look-up table updating will be required. The third of these parts, area modulation versus continuous tone recording, will to be described in more detail below. Lastly, the fourth of these parts, density uniformity, refers to the ability of a hardcopy device to generate a uniform, flat field over the entire image area.
A continuous tone recording has an apparent continuum of gray scale levels such as are observed, for example, in photographs and in natural scenes. This is contrasted with an area modulation recording which is typically comprised of geometric patterns of, for example, printed dots--please note that printing with patterns of variable-sized dots is frequently referred to as halftone recording in the art. In halftone recording, the printed dot size in a regular array is varied to provide a range of tones perceived as a gray scale by the human eye.
As is well known to those of ordinary skill in the art, a continuous gray scale may be approximated in halftone recording because variations in printed dot size yield, for example, a varying percentage of light reflection from a printed image and, as a result, create an illusion of a gray scale. Although halftone recording is basically binary, at first blush, one would expect a halftone recording image to be like that of a line copy.
However, halftone recording is complicated by the presence of spatial frequencies which are not contained in the original image, which spatial frequencies may result in unwanted Moire patterns or other artifacts in the halftone recording image.
As disclosed in the prior art, in one halftone recording method for achieving gray scale representations by binary devices, i.e., devices which display or print fixed size dots having no gray scale capability, each halftone cell, herein denoted as a pixel, is comprised of one or more clusters of individual print or display units, herein denoted as pels. The most common form of halftone pixel is an N by N square pel matrix of binary, fixed sized pels. The general concept of the method is to print or display a computed number of pels within a halftone pixel to achieve an average gray scale level which approximates the averaged density value of a corresponding portion of the original image. For example, in one such prior art halftone recording method, pels in a pixel are clustered to imitate the formation of a single halftone pixel and, in another such prior art halftone recording method, pels are dispersed in a predetermined manner. Further, in still another such prior art halftone recording method, referred to as "error diffusion," a decision to print or not to print a pel is made on the basis of local scanned density information from the original image as well as on gray scale density errors committed by already processed neighbors in the recording. In addition to the above, those of ordinary skill in the art appreciate that while halftone recording reproduces gray scale levels for a pixel in an averaged sense, there may be a loss of fine detail resolution in an image if the size of the pixel is too large.
All of the above-mentioned prior art halftone recording methods disclose the use of binary, fixed size, print or display dots. In contrast to this, U.S. Pat. No. 4,651,287 discloses a halftone recording method in which each picture element to be printed or displayed is programmably adjusted to have one of a fixed number of gray scale levels. The patent discloses a halftone recording apparatus which includes: (a) image data input apparatus such as, for example, a CCD scanner for scanning an original image and for producing an array of image input data corresponding to gray scale levels of picture elements of the original image; (b) processing apparatus for receiving the array of image input data and for computing an array of print values wherein each print value corresponds to one of a fixed number of gray scale levels; and (c) printing apparatus capable of printing picture elements having a dot size that corresponds to one of the fixed gray scale levels.
In addition, the patent discloses that a printer which is capable of printing picture elements wherein each picture element has a dot size that corresponds to one of a fixed number of gray scale levels may include apparatus which varies the energy necessary for the production of a printed dot. Further, the patent discloses that the energy necessary for the production of a printed dot is generally prescribed in the form of an electrical signal pulse having a predetermined time duration and a predetermined voltage level. Lastly, the patent discloses that variations of the energy can be affected by changing the following parameters of the electrical signal pulse: the on-time portion (duty cycle); the voltage level; or the electrical current flow.
U.S. Pat. No. 4,661,859 discloses an apparatus which produces a pixel having a variable gray scale. In particular, it discloses a one-dimensional electronic halftone generating system which is comprised of a source of digital data representative of pixel gray scale, a counter to store the digital data, and pulse producing logic responsive to the counter to activate a laser modulator in accordance with the digital data representative of each pixel. More particularly, a six bit data word is used to represent one of 64 gray scale levels for a pixel, and the pulse producing logic responds to the data word by producing a pulse of a predetermined duration or width which drives the laser for a predetermined time duration to produce a predetermined gray scale level for the pixel.
Notwithstanding the above prior art halftone recording methods and apparatus, there still remains a need in the art for method(s) and apparatus which can provide a faithful reproduction of an image rapidly, which method and apparatus include strong gray scale sensitivity without sacrificing resolution and which method and apparatus are particularly suitable for providing a reproduction of an image which is generated or acquired from medical imaging equipment such as x-ray equipment, CAT scan equipment, MR equipment, ultrasound equipment, and the like.